U.S. Pat. No. 4,429,990, issued Feb. 7, 1984 to E. J. Tamary, shows an electrophotographic copier which forms a series of toner images. The copier has two transfer stations. In a simplex mode, one transfer station is used to transfer toner images to a single side of receiving sheets which are then fed to a fuser for fixing. In a duplex mode, receiving sheets are fed to a first transfer station for receiving toner images on one side, immediately turned over, fed to a second transfer station to receive a toner image on the other side and then fed to the fuser where both images are fused simultaneously.
The machine shown in the Tamary patent has been successful commercially. It has the very important advantage of providing duplex output without passing the copy sheet through the fuser twice. It is termed a "single pass" duplexing apparatus. It is contrasted with "double pass" duplexing systems in which a series of sheets receive one image, are fused and placed in an intermediate tray. They are fed back to the image member to receive the second image and pass through the fuser again. One image receives twice the fusing of the other and the sheet is heated twice.
Although the advantages of "single pass" duplexing are many, the task of fusing duplex images in one mode and simplex images in another mode while controlling the surface temperatures of the fusing members is challenging. The commercial duplex fuser utilizing the Tamary technology has a pair of fusing rollers which are each heated from within. The fusing roller which contacts the side of a receiving sheet carrying a simplex image (called the "simplex roller") has a thick elastomeric covering on a metal core.
The other fusing roller (the "duplex roller") has a thin elastomeric covering also on a metal core. Both fusing rollers have temperature control devices which sense the surface temperature of the metal core outside the image area to control the power applied to internal heating lamps within the rollers. Because of the thickness of the elastomer on the roller contacting the simplex side of the receiving sheets, a relatively high set point must be used for the metal core of that roller. This is because the temperature of the exterior surface of the elastomer for any given roller with a constant heat source decreases as a function of the thickness of the elastomer.
Although this apparatus produces excellent images, the power consumption of the fuser is substantial, even working with relatively low fusing temperature toners. Further, while the core temperature of the simplex roller is not too high using toners fusable at 340.degree. F. for which it is designed, to use higher fusing temperature toners (for example, 380.degree. F.) the core temperature leaves little upper latitude with respect to a high shut off point for the fuser and the char point of paper. Even at lower fusing temperatures, throughput is limited by the limit on the core temperature. Additionally, the thick outer layer causes a droop in temperature at the beginning of a run requiring a toner that fuses over a wide range of temperatures.
A number of references show simplex fusers which include heating lamps in both rollers. The heating lamp in the roller that does not contact the image is generally used to prevent that roller from lowering the temperature of the roller which does contact the image when the rollers are in contact between images. See, for example, U.S. Pat. No. 4,231,653, Nagahara et al, issued Nov. 4, 1980; U.S. Pat. No. 4,549,803, Ohno et al, Oct. 29, 1985; U.S. Pat. No. 4,595,274, Sakurai, Jun. 17, 1986; U.S. Pat. No. 4,618,240, Sakurai et al, Oct. 21, 1986; U.S. Pat. No. 4,019,024 Namiki, Apr. 19, 1977; U.S. Pat. No. 3,945,726, Ito et al, Mar. 23, 1976 and U.S. Pat. No. 3,268,351, VanDorn, Aug. 23, 1966.